The transistor's work function can be modulated using dopants in combination with fully silicided gates. The use of an activation anneal of these dopants has been shown to have a strong effect on the work function modulation achieved. Aluminum (Al) can be used as dopant to fabricate poly-Si gate for p-MOS transistors whereas Ytterbium (Yb) doped poly-Si gate will be used for n-MOS transistors. In the complementary MOS (CMOS) transistor flow, the gates might receive the implants (Yb and Al) before the gate patterning or just before the silicidation. In the former case they can receive the anneal from the junction anneal, whereas in the latter case they will not get an activation anneal because the integrity of the NiSi source drain regions could not be preserved.
Since Yb is not likely to form volatile compounds during dry etching the problem is the selective etching of Yb doped gates without leaving residues leading to unwanted micro-masking.